How can I be sure that students don’t share accounts?

Students use their credentials to verify their affiliation status. As explained in this website InAcademia redirects users / students to the authentication section of their university or higher education institute. By entering their credentials they prove to be legitimate students.

The same credentials of the student are used to plan exams, upload theses and assignments, sign up for classes and present the grades a student got. Even student loans are requested by using the student’s credentials. We seriously doubt that any student would give these credentials to other persons. It’s very much in the self-interest of users to not share their accounts.

However, we can not guarantee this.
We do know for sure that by definition the owner of the credentials is a student. It is the obligation of a university that once a student has finished his exams and the students stops with his courses, to unsubscribe the student from its records. This usually is done immediately after finishing.